Mass spectrometry



June 26, 1956 R. L. SINK 2,752,502

MASS SPECTROMETRY Filed April 16, 1953 2 Sheets5heet 1 REFERENCE VOLTAGE ACCELERAT/NG VOLTAGE POWER SUPPLY PHASE SENS/N6 AMPL /F /E R 8 RECORDER CVCL/C SCAN VOLTAGE SUPER/M- POSED o/v ACCELERAT/NG VOLTAGE L PROPERLY ADJUSTED ACCELERAT- b) '1 ING VOLTAGE MAX. AMPL/TUDE= T T ACCELERAT/NG VOLTAGE r T 700 HIGH L 69 T ACCELERAT/NG VOLTAGE T 7'00 LOW INVENTOR. ROBERT L SINK ATTORNEY June 26, 1956 R. SINK 2,752,502

MASS SPECTROMETRY Filed April 16, 1953 2 Sheets-Sheet 2 T W WHHFFP" I UH! i INVENTOR. ROBERT L. SINK A T TORNE Y United States Patent MASS SPECTRONIETRY Robert L. Sink, Aitadena, Califi, assignor, by mesne assignments, to Consolidated Electrodynamics Corporation, Pasadena, Calif., a corporation of California Application April 16, 1953, Serial No. 349,161

15 Claims. (Cl. zen-41.9

This invention relates to mass spectrometry and more particularly to a method of automatically holding a mass spectrometer centered on a given peak or mass ratio reading independently of variations in accelerating voltage power supply, or magnetic field strength. The method is particularly suited for use in conjunction with isotoperatio mass spectrometry and, although in no way so limited, is described in this connection.

A mass spectrometer is an analytical instrument for sorting and measuring ions. Ordinarily, it includes an ionization chamber in which molecules of a sample to be analyzed are bombarded with a stream of electrons to convert them into ions. The ions thus formed are expelled from the ionization chamber and are propelled by so-called accelerating electrodes into and through an analyzer chamber. During passage through the analyzer chamber, the heterogeneous beam of ions is subjected to a transverse magnetic field to separate it into diverging ion beams, each of which is composed of ions of a given mass-to-charge ratio which diflers from the massto-charge ratio of the ions forming the other beams. The diverging beams may be successively focused through an exit or resolving slit in a terminal electrode onto a collector electrode. Alternatively, a single one of the ion beams may be continuously focused on the resolving slit to the exclusion of the other beams. The current produced by ion discharge at the collector electrode is indicative of the amount of ions in the particular beam and hence constitutes a measure of the partial pressure of the molecules (from which the ions were derived) in the sample being analyzed.

A specialized form of mass spectrometry involves the measurement of the ratio of isotopes in a sample. In a mass spectrometer used for this purpose a pair of collector electrodes are employed. One of these is referred to as the low current electrode, and at this electrode the ion beam derived from molecules of a low abundant isotope is discharged. The other electrode is referred to as the high current electrode and the ion beam or beams of the more abundant isotope or isotopes are focused on and discharged at this electrode. In isotope-ratio analysis it is generally desired to measure the ratio of the ion current rather than the absolute value of each of the components. Accordingly, the amplification and recording circuit connected to the two collector electrodes is frequently arranged in a null balance network so that the current developed at one collector is eifectively divided by the current developed at the other collector to give a recorded value representative of the indicated ratio. Amplification and recording circuits of this character are illustrated and described in U. S. Patent 2,456,426 issued to Alfred O. C. Nier on December 14, 1948, and in U. S. patent application Serial No. 104,030, filed by me on July 1 l, 1949.

In the mass analysis of isotope ratios or in the monitoring of a single ion peak, the value of the accelerating voltage power supply and of the magnetic field as developed by an electromagnet must be held to very close.

2,752,502 Patented June 26, 1956 limits if true values are to be recorded. As an example of the accuracy required in this respect, it has been determined in a typical isotope-ratio operation that the magnet current must be constant within 0.002% for a period of at least 15 minutes to assure satisfactory measurement of the ratio. The high voltage power supply must also remain constant within 0.002% in the same period of time. This accuracy requirement is nearly one order of magnitude beyond that which is normally considered practical or possible. It is evident that unattendedinstruments cannot achieve this accuracy over any extended period of time. It is more practical, therefore, to consider the automatic adjustment of either the accelerating voltage or magnet current than to attempt to provide increased stability thereof.

The invention is directed to a method and apparatus for assuring that an isotope-ratio measurement or an ion peak measurement in a monitoring application be correct even though the equipment may be unattended over considerable periods of time. In one aspect the invention'contemplates in the method of mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means of a D. C. accelerating potential into an analyzing zone as a heterogeneous beam, subjecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams and discharging at least one of said diverging beams to develop a detectable signal, the improvement which comprises superimposing on the D. C. accelerating potential an oscillatory voltage whereby said detectable signal is caused to oscillate in amplitude, and varying either the accelerating voltage or magnet current as required to maintain substantially symmetrical oscillation of the signal about an average value.

There are many means of implementing the method as described above, one preferred means being illustrated and described herein as forming a part of this invention. To this effect the invention contemplates in a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes for propelling ions from the ionization chamber into and through the analyzer chamber, means for establishing a transverse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential, and means for recording the relative magnitude of the signal developed by such collection, the combination comprising means for superimposing on the accelerating potential an oscillating potential whereby the recorded signal is caused to oscillate, and means operable responsive to unsymmetrical oscillation of the recorded signal to adjust either the accelerating potential or the magnetic field strength in a direction to return the recorded signal to symmetrical oscillation.

By impressing an oscillating signal, as for example a linearly varying saw tooth configuration, on the accelerating potential in an isotope-ratio mass spectrometer, the. indicated isotope-ratio will oscillate about an average value. If the accelerating voltage is such as to properly focus the isotopes of interest on the respective collectors, the oscillation of the recorded trace is symmetrical about this average value. As a consequence of variation in the power supply determinative of the accelerating potential or variation in the magnetic field strength, the oscillation of the recorded trace will become unsymmetrical, which loss of symmetry will be reflected in greater angular displacement of the recorder drive motor shaft. I have made use of this feature to operate a switch connected to an auxiliary motor adapted to adjust the accelerating potential independent of the high voltage power supply, the system including phase-sensitive means whereby the operation of the auxiliary-adjusting motor is in the proper direction to return the recorded trace to symmetrical. oscillation.

The invention will be more clearly understood withreference to the following. detailed description taken in. conjunctionwith the accompanying-drawing, in which:

Fig. l is a schematic circuit. diagram of. one embodiment of the invention;

Fig. 2 is a graphic portrayal illustrating the principles of the invention;

Fig. 3 is a reproduction of an actual record. showing theoperation of the invention;

Fig. .4.is. an elevation. view of a recorder. operated switch for. effecting. energization of the. accelerating. voltageradjustingmotor; and

Fig. 5 is a longitudinal sectional elevation taken on the line 5-5 of Fig. 4.

InFig. 1 portions. of a mass spectrometer are shown schematically including an ion source 11 having a sample: inletlZ, accelerating electrodes 13, 14, an analyzer tube 15., and an isotope-ratio collecting system including a resolving electrode 16, a first collector electrode 17 and a second. collector electrode 18. The two collector electrodes 17 and 18 are connected in conventional fashion to. an. amplifying and recording system 20. The entire mass spectrometer tube is conventionally immersed in a transverse, magnetic field developed by opposing magnet poles, magnet pole 22 being illustrated in the. drawing. An. electron beam 24 is projected across the ion source 11. by. an electron. gun (not shown) and functions to ionize. molecules of a sample introduced to the ion source through inlet 12'. The operation of a mass spectrometer is well knownand requires no detailed description.

The. potentials applied to the accelerating electrodes for propelling ions from theion source as a heterogeneous beamintothe analyzer tube are derived from an accelerating voltage power supply 26, the character of which forms no part of the present invention. Many types of high. voltage power supplies have been designed and are available. for such purpose. The output of the power supply 26 is applied across avoltage divider network 26A including. resistors 27, 28, 29, an adjustable fraction of the voltage appearing across the center resistor 28 being tapped oil by adjustable tap 30 and applied across a second. divider network 31 including a resistor 32. The accelerating electrodes 13 and 14 are connected to the secondary voltage divider 31 as desired to develop the proper relationship of. the potentials impressed thereon.

Any regulated. high voltage power supply is provided with a source of reference voltage to stabilize the high voltage. output of the supply. In accordance with the present invention this source comprises a battery 34 and a so-called. cyclic scan. potentiometer 36 driven, by a potentiometer motor 38. A voltage source illustrated as battery 40 is connected through a voltage divider network. 42'. across the potentiometer. Opposite potentiometer leads are connected through the battery 34 to the high voltage power supply and the. variable tap 36B. of the potentiometer is connected independently to the high voltage power supply. As a consequence the reference" voltage. to the power supply is varied responsive to rota-- inseriesbetween a switch 59 and an accelerating voltage-- adjusting motor 52. The construction of. the cam' 47 and switch 48 is illustrated in greater detail in Figs; 4 and.5 and described in relation thereto.

Switch 50. is a. single-pole, double-throw switch andiS:

4 connected to be actuated responsive to a phase-sensing cam 54' which is driven in continuous rotation by the potentiometer motor 38, the cam 54 rotating once for each rotation of the tap 36B of the cyclic scan potentiometer. The switch 50 is connected through switch 48 to the field coils of the accelerating voltage adjusting motor 52 whereby the motor 52 is operated responsive to actuation of switch 48 and in a direction determined by the position of switch 50. The system including the cam 54 and switch 50 provides phase sense to the net'- work so that reversible motor 52 will operate only in a direction to provide the required correction. The motor 52 is in turn connected to variable tap 30 of the accelerating voltage divider 26A. for varying the voltage applied across divider 31 responsive to actuation of the motor 52.

The operation of the apparatus is described with relation to Figs. 2 and 3. In Fig. 2 curve A illustrates the character of the voltage superimposed on the accelerating voltage as a consequence of the continuous. rotation of the cyclic scan potentiometer 36 which etfectuates a corresponding variation in the reference voltage. If'the accelerating voltage is properly adjusted so as to focus the proper ion beams on the respective collector electrodes, the recorded ratio will have the character of curve B of Fig. 2 oscillating symmetrically about a median line with a maximum amplitude of oscillation of T as illustrated. If the accelerating voltage is too high so that ion beam focus is distorted in the direction of greater ion travel radius, the recorded isotope-ratio will have the character of curve C of Fig. 2 with alternate peaks of oscillation being of greater amplitude than T and: intermediate peaks being of correspondingly smaller amplitudes. If the accelerating voltage is too low so that focus is disturbed in the direction of smaller ion beam radius, the recorded isotope-ratio will havethe character of curve D of Fig. 2, again with alternate peaks of oscillation having an amplitude in excess of T and intermediate peaks having correspondingly smaller amplitudes. The disturbance in this instance being out of phase with that occurring at a too high accelerating voltage (curve C).

In the. operation of the apparatus of Fig. 1 and under the circumstances illustrated in curves C and D of Fig. 2, rotation. of cam 47 connected to the recorder motor shaft 46 to the extent induced by the fact of pen excursion in excess of'amplitude T, closes switch 48 thereby conmeeting the phase-sensing system with the accelerating voltage adjusting motor 52. Cam 54 actuates switch 50 to energize appropriate ones of the field windings of the motor. 52. so as to adjust tap 30 of the divider network 26A in the proper direction to correct the accelerating voltage so as.-to return the recorded trace to symmetrical oscillation. The phase-sensing cam 54 is developed to actuate the switch 50 at the appropriate portions of the unsymmetrical oscillations, i. e. when the switch 48 is closed: as a consequence of such unsymmetrical oscillation of the recorded trace, so that the direction of rotationofthe motor 52 is correlated with the nature of the lack of symmetry of such oscillation. By way of example, if' the accelerating voltage is too high with the consequent trace characteristics, as illustrated in curve C of Fig. 2, then switch 43. is closed during a part of the period in;whichi cam. 54 closes switch 50 in one direction or another, switch 48 opening again before switch 50 is oppositely actuatedv responsive to continued rotation of the cam. Conversely, if the accelerating voltage is too low, switch 48 is closed during the period of such opposite actuation of switch 54 As a consequence, the system is entirely phase-sensitive and corrective adjustment inthe:accelerating'voltage is made only during the periods of trace excursionbeyond the symmetrical amplitude: T and only in the proper direction dependent. upon the phase of such unsymmetrical oscillation.

As will be described in greater detail with respectto- Eigs;.4 and 5, the cam: 47 which controls the actuation.

of switch 48 and hence the adjusting motor 52 is'connected through suitable clutch means to the recorder motor shaft 46. Such connection is essential so that the system will accommodate a true variation in the function to be measured. In other words, the system must be adapted to correct for spurious variations in output signal whether such signal be in the nature of a ratio measurement or a monitored peak height and at the same time to tolerate analytically significant variations of such signals.

Assume the accelerating voltage to be properly adjusted for correct focus of the ion beams of interest, when a change in the ratio of the isotopes being measured is reflected in an excursion of the recorder pen outside of the so-called symmetrical amplitude T, the corrective system will be energized as described. However, in a very brief interval it automatically returns to symmetrical oscillation at the same amplitude T and about a different average point so that the new ratio is immediately reflected. An actual recorded trace is shown in Fig. 3. Reading from left to right of Fig. 3 the section F represents symmetrical oscillation of the trace about the correctly indicated isotope-ratio X and at an amplitude T. If either the magnetic field or the accelerating voltage supply are disturbed, and in this instance the disturbance was artificially induced for illustrative purposes, the oscillation becomes unsymmetrical and with every other peak falling well below the lower limit of the symmetrical amplitude T and the intermediate peaks being of correspondingly small amplitude. Under such circumstances the system illustrated in Fig. 1 immediately commences corrective adjustment, the section G of the trace representing such an adjustive period. As the excursions of the pen beyond the lower limit of symmetrical amplitude T are brought back to this limit, the intervening incomplete excursions are similarly brought back to full oscillation by automatic variation of the accelerating voltage to correct for the artificially induced variation. Now if the ratio being measured is varied, much the same effect takes place, it being noted that the oscillation immediately becomes unsymmetrical with alternate peaks extending well below the previously established symmetrical region and the intervening peaks being practically obscured. This eifect sets the corrective system into action and automatically and quickly returns itself to symmetrical oscillation about the new correct isotoperatio Y. The section I of the curve represents the period of restoration to symmetry and the section I of the curve represents the symmetrical oscillation about a new correct value of isotope-ratio and again with the same amplitude T as determined in each instance by the oscillating voltage superimposed on the D. C. accelerating voltage.

The recorder-actuated cam and switch mechanism is illustrated in greater detail in elevation in Fig. 4 and longitudinal section in Fig. 5. A tubular extension 60 is fastened on the recorder motor shaft 62 by means of a lock collar 63, the extension 60 being carried through a bushing 64 in the recorder housing 66. A mounting plate 68 is fastened on the bushing 64 exteriorly of the housing 66 and supports a microswitch 70 actuated by a depending cam follower 72. The support plate 68 has a semi-circular groove or slot 74 formed adjacent its lower end in which a pair of stops 75, 76 are adjustably mounted. The stops may be positioned anywhere around the semi-circular slot 74. A sleeve 78 is aflixed on the tube 60 and held by a set screw 79, the sleeve having an annular radially extending flange 78A at one end against which a cam 82 is mounted, the cam being rotatable about the sleeve 78. An annular end plate 84 is afilxed to the opposite end of the sleeve 78 as by a screw 85 and a helical spring 88 is confined between the end plate 84 and the cam 82, there being an intervening washer 90 to permit rotation of the cam independently of the spring 88. A pin 92 extends inwardly from the cam for engagement with stops 75, 76car'ried by the supporting plate 68.

With symmetrical oscillation of the signal applied to the recorder, pin 92 will oscillate in a region of smaller angular inclusion than the stops 75, 76 as for example the region represented by arrow Z. If oscillation of the signal to the recording motor becomes unsymmetrical, the angular excursions of the pin 92 will increase in the direction of stop 76, and as the pin approaches stop 76 the cam follower 72 will be deflected by the shorter radius portion of the cam surface to actuate switch 70 and initiate corrective action. The limit of angular excursion of the pin 92 is set by the stops and if the deviation from symmetrical oscillation is in excess of the limited angular excursion of the pin the clutch arrangement described permits cam 82 to slip on the shaft 60. In this manner the system accommodates a change in absolute value of the signal being measured and will automatically return itself to symmetrical oscillation of the signal as previously described and corresponding symmetrical oscillation of pin 92 in the predetermined angular range intermediate the stops 75, 76.

As illustrated in Fig. 5 the tubular extension 60 on the motor shaft 62 is normally carried beyond the cam and clutch system illustrated for other purposes not germane to this invention.

The illustrated apparatus comprises preferred means for accomplishing the method of the invention. However, the method can also be carried out by adjustment of magnetic field strength either by means of variable shunts or current control to an electromagnet responsive to unsymmetrical oscillation of the ion beam. Similarly one preferred means of inducing ion beam oscillation is illustrated in the motor driven potentiometer 36 (Fig. 1) included in the reference voltage network of the regulated power supply. Other means, not limited to the development of a saw tooth signal, may be used to induce oscillation of the beam. An auxiliary source of oscillating voltage may, for example, be connected to superimpose a signal of such character on the voltage applied to the accelerating electrodes. In addition a separate modulating electrode may be embodied in the ion source for accomplishing the same efiect. In short, many forms of apparatus will occur to those skilled in the art for practicing the peak holding method as described.

I claim:

1. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes for propelling ions from the ionization chamber into and through the analyzer chamber, means for establishing a transverse magnetic field in the analyzer chamber control means for selectively focusing at least one ion beam on a collector electrode and means for recording the relative magnitude of the signal developed by discharge of the ions of said beam at the collector, the combination comprising means for oscillating the ion beam whereby the recorded signal is caused to oscillate, and means operable responsive to unsymmetrical oscillation of the recorded signal to adjust the control means in a direction to return the recorded signal to symmetrical oscillation.

2. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes for propelling ions from the ionization chamber into and through the analyzer chamber, means for establishing a transverse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential, and means for recording the relative magnitude of the signal developed by such collection, the combination comprising means for causing the accelerating potential to oscillate whereby the recorded signal is caused to oscillate, and means operable responsive to unsymmetrical oscillation of the recorded signal to adjust the accelerating potential in :a direction to return the-recorded'signal to symmetrical oscillation.

3. In a massspe'ctrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage, means connected to impress a fraction of the D. C. voltage developed at said source on the accelerating electrodes to establish an acceleratingpotential between the accelerating electrodes for propelling ions from the ionization chamber into and through the analyzer ch-amber, means for establishing a transverse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential, and means for recording the relative magnitude of the signal developed by such collection, the combination comprising means for causing the accelerating potential to oscillate whereby the recorded signal is caused to oscillate, and means operable responsive to unsymmetrical oscillation of the recorded signal to adjust the fraction of said D. C. voltage impressed on the electrodes to return the recorded signal to symmetrical oscillation.

4. -In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, at regulated power supply connected to impress a variable accelerating potential on the accelerating electrodes for propelling ions from the ionization chamber into and through the analyzer chamber, means for selectively collect-ing at least one ion beam at a given accelerating potential, and means for recording the relative magnitude of the signal developed by 'such colleo'tion, the combination comprising means for causing the accelerating potential to oscillate whereby the recorded signal is caused to oscillate, and means operable responsive to unsymmetrical oscillation of the recorded signal to adjust the accelerating potential in a direction to return the recorded signal to symmetrical oscillation.

5. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a regulated power supply connected to impress a variable accelerating potential on the accelerating electrodes for propelling ions from the ionization chamber into and through the analyzer chamber, means for establishing atransv-erse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential, and means for recording the relative magnitude of the signal developed by such collection, the combination comprising means for causing the accelerating potential to oscillate whereby the recorded signal is caused to oscillate, means operable responsive to unsymmetrical oscillation of the recorded signal to adjust the accelerating potential and phase sensitive means connected to determine the adjustment of the accelerating potential in a direction to return the recorded signal to symmetrical oscillation.

'6. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, at regulated power supply for impressing an accelerating potential on the accelerating electrodes and including a reference voltage source, means for establishing a trans verse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential, and recording means including a motor the shaft of which rotates responsive to variation in'th'e magnitude of the electrical signal developed by ion collection, the combination comprising means operable to oscillate the reference voltage source, adjusting means operable to vary the proportion of the regulated output of the regulated power supply impressed on the accelerating electrodes, and means sensitive to angular displacement of the recorder motor shaft in excess of a given amount to actuate the adjusting means.

7. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a regulated power supply for impressing an accelerating potential :on' the accelerating electrodes and including a reference voltage source, means for establishing a transverse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential and recording means including .a first motor the shaft of which rotates responsive to variation in the magnitude of the electrical signal developed by collection of the ion beam, the combination comprising a potentiometer connected to said reference voltage source, a second motor connected to uniformly vary the setting of the potentiometer in repeating cycles to cause oscillation of the reference voltage, means operable to vary the proportion of the regulated outputxof the regulated power supply impressed on the accelerating electrodes, and means sensitive to angular displacement of the recorder motor shaft in excess of a given amount to actuate the adjusting means.

8. In a mass spectrometer having an analyzer chamber, accelerating electrodes, a regulated power supply for impressing an accelerating potential on the accelerating electrodes and including a reference voltage source, means for establishing a transverse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential and recordin means including a first motor the shaft of which rotates responsive to variation in the magnitude of the electrical signal developed by collection of the ion beam, the combination comprising a potentiometer connected to said reference voltage source, a second motor connected to uniformly vary the setting of the potentiometer in repeating cycles to cause oscillation of the reference voltage, a third motor operable to vary the proportion of the regulated output of the regulated power supply impressed on the accelerating electrodes, and means sensitive to angular displacement of the shaft of the first motor in excess of a given amount to actuate the third motor.

9. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a regulatedpower supply for impressing an accelerating potential on the accelerating electrodes and including a reference voltage source, means for establishing a transverse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential and recording means including a first motor the shaft of which rotates responsive to variation in the magnitude of the electrical signal developed by collection of the ion beam, the combination comprising a potentiometer connected to uniformly vary the setting of'the potentiometer in repeating cycles to cause oscillation of the reference voltage, a third reversible motor operable to vary the proportion of the regulated output of the regulated power supply impressed on the accelerating electrodes, a first switch actuated responsive to angular displacement of the shaft of the first motor in excess of a given amount, a second dipole switch actuated responsive to the setting of the potentiometer, the first and second switches being in circuit with the third motor, the first switch determining actuation thereof and the second switch determining the direction of rotation'thereof when actuated.

10. In amass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a regulated power supply for impressing an accelerating potential on the accelerating electrodes and including a reference voltage source, means for establishing a transverse magnetic field in the analyzer chamber, means for selectively collecting at least one ion beam at a given accelerating potential and recording means including a first motor the shaft of which rotates responsive to variation in the magnitude of the electrical signal developed by collection of the ion beam, the combination comprising a potentiometer connected to said reference voltage source, a second motor connected to uniformly vary the setting of the potentiometer in repeating cycles to cause oscillation of the reference voltage, a third reversible motor operable to vary the proportion of the regulated output of the regulated power supply impressed on the accelerating electrodes, a first switch, a second dipole switch connected to be actuated responsive to the setting of the potentiometer, the first and second switches being connected in circuit with the third motor, the first switch determining actuation thereof, the second switch determining direction of rotation, and displacement means sensitive to angular displacement of the shaft of the first motor in excess of a given amount to close the first switch thereby actuating the third motor.

11. Apparatus according to claim wherein the displacement means comprises a cam mounted on the first motor shaft and engaging the first switch, a slip clutch linkage connecting the cam to the shaft and adjustable limit means limiting the angular displacement of the cam, the cam surface being developed to actuate the first switch responsive to angular displacement in excess of a predetermined amount less than the limited angular displacement.

12. In a mass spectrometer having means for ionizing a sample to be analyzed, means for propelling the ions by means of a D. C. accelerating potential into an analyzing zone as a heterogeneous beam, means for subjecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogenous beam into a plurality of diverging beams, and means for focusing at least one of said diverging beams to discharge on a collector electrode and thereby develop a detectable signal, the improvement which comprises means for causing the ion beam to oscillate whereby said detectable signal is caused to oscillate, and means for varying the focus of the ion beam to maintain substantially symmetrical oscillation of the signal about an average value.

13. In a mass spectrometer having means for ionizing a sample to be analyzed, means for propelling the ions by means of a D. C. accelerating potential into an analyzing zone as a heterogeneous beam, means for subjecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams, and means for focusing at least one of said diverging beams to discharge on a collector electrode and thereby develop a detectable signal, the improvement which comprises means for causing the D. C. accelerating potential to oscillate whereby said detectable signal is caused to oscillate, and means for varying the focus of the ion beam to maintain substantially symmetrical oscillation of the signal about an average value.

14. In a mass spectrometer having means for ionizing a sample to be analyzed, means for propelling the ions by means of a D. C. accelerating potential into an analyzing zone as a heterogeneous beam, means for subjecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams, and means for discharging at least one of said diverging beams to develop a detectable signal, the improvement which comprises means for causing the D. C. accelerating potential to oscillate whereby said detectable signal is caused to oscillate, and means for controlling the accelerating voltage to maintain substantially symmetrical oscillation of the signal about an average value.

15. In a mass spectrometer having means for ionizing a sample to be analyzed, means for propelling the ions by means of a D. C. accelerating potential into an analyzing zone as a heterogeneous beam, means for subjecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams, means for discharging at least one of said diverging beams to develop a detectable signal and recording the signal as a continuous trace, the improvement which comprises means for causing the D. C. accelerating potentialto oscillate whereby said detectable signal is caused to oscillate, and means for controlling the accelerating voltage to maintain substantially symmetrical oscillation of the recorded trace.

No references cited. 

1. IN A MASS SPECTROMETER HAVING AN ANALYSER CHAMBER, IONIZATION CHAMBER, ACCELERATING ELECTRODES, A SOURCE OF D. C. VOLTAGE CONNECTED TO IMPRESS AN ACCELERATING POTENTIAL ON THE ACCELERATING ELECTRODES FOR PROPELLING IONS FROM THE IONIZATION CHAMBER INTO AND THROUGH THE ANALYZER CHAMBER, MEANS FOR ESTABLISHING A TRANSVERSE MAGNETIC FIELD IN THE ANALYZER CHAMBER CONTROL MEANS FOR SELECTIVELY FOCUSING AT LEAST ONE ION BEAM ON A COLLECTOR ELECTRODE AND MEANS FOR RECORDING THE RELATIVE MAGNITUDE OF THE SIGNAL DEVELOPED BY DISCHARGE OF THE IONS OF SAID BEAM AT THE COLLECTOR, THE COMBINATION COMPRISING MEANS OF OSCILLATING THE ION BEAM WHEREBY THE RECORDED SIGNAL IS CAUSED TO OSCILLATE, AND MEANS OPERABLE RESPONSIVE TO UNSYMMETRICAL OSCILLATION OF THE RECORDED SIGNAL TO ADJUST THE CONTROL MEANS IN A DIRECTION OF RETURN THE RECORDED SIGNAL TO SYMMETRICAL OSCILLATION. 